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Help: Pioneer receiver repair?

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Greg Dunn

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May 7, 1998, 3:00:00 AM5/7/98
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I'll probably be able to track this down eventually, but if anyone
has an "Aha!" insight, please feel free to share it with me. :-)

I'm trying to repair a nice old Pioneer SX-737 receiver that I got
for pennies on the dollar at a flea market; it works great except
that the right channel high level circuitry appears to have an
intermittent DC offset problem. Listening to the output, the thing
manifests itself as a static-like noise followed by several loud
thumps. Then it settles down for a while. The protection relay
(driven by a small circuit board connected directly to the output
transistors) clicks 2-3 times a minute, but not in synchronism with
the thumping. I've measured the output terminals, and there does
seem to be a pronounced DC transient associated with the sound
(say, 2-5 V).

The high-level and low-level circuits are on separate circuit
boards, and the power section seems to be a standard differential
input->complementary driver->complementary output topology, but I
have no schematic (maybe I should order one, though it'll cost more
than I spent on the receiver :-) There are no obviously broken or
overheated components. Power supply voltage is +/-35V to the
output stage, and seems steady.

I've re-flowed a few suspect solder joints on the output board,
and cautiously checked for physical sensitivity of obvious
components (translation: tapped transistors and resistors with an
insulating stick) but nothing seems to provoke or stop the noise.
It's definitely isolated to the right channel (I pulled the power
supply fuses for that channel to check) but seems to be hiding
well. It's not heat-related, as it starts almost immediately after
turn-on and doesn't seem to get better or worse with time.
Defective capacitor in the output stage? Output transistor
shorting through the mica insulator to the heat sink?

Anyone? Anyone? Bueller? :-)


--
| Greg Dunn | And did your mother lie to you? |
| Greg...@aol.com | You got God and death confused |
| greg...@indy.net | I can't blame it on your soul |
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| http://members.aol.com/gregdunn | Heather Nova |

BEACHMINTZ

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May 7, 1998, 3:00:00 AM5/7/98
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You will find either a dual transistor (2SA799 or similar) or a pair of PNP
transistors (2SA733 or similar) at the input stage of the power amp section.
This is a differential pair configuration and is known to cause that problem.
The transistors will check good but replace them. Do both channels while you
are at it or you will have to do it again soon.

Jerry G.

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May 7, 1998, 3:00:00 AM5/7/98
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Hi there...

I have had bad caps, driver and output transistors cause such problems in
these amps. I once changed all the caps and transistors in the final of a
one of these to fix it. It cured it for years!

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Mzacharias

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May 8, 1998, 3:00:00 AM5/8/98
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Could be either differential-pair transistors at the input area of the main
amp, or bad (probably Sanyo) capacitors. A can of freeze-mist might help
narrow it down.

Mark Z.
Wichita KS.

alan

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May 8, 1998, 3:00:00 AM5/8/98
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Usually the older pioneers that exhibit this type of problem have some
leaky or noisy transistors back in the driver stages and since they
usually have direct coupling between stages,it is passed all the way to
the outputs.A can of freeze spray can sometimes pinpoint the noisy ones
but you have to be careful,this method can sometimes cause dc transients
that can wipe out the whole channel from driver to final or at the very
least pop a voice coil on the speaker if the protect relay don't open
fast enough.
good luck.

NikonKidF3

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May 9, 1998, 3:00:00 AM5/9/98
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Try resoldering all the power transistors and other devices that have large
leads. I have found that these devices will develop bad solder joints because
of the repeated heating-cooling cycles produced during normal operation.

Willis

Willis Chung
1207 Canvasback Court
Fort Collins, CO 80525
970-204-4838h 970-350-6860w

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